It is well known that the efficiency of catalyst systems is often related to the surface area on the support. This is especially true for systems using precious metal catalysts or other expensive catalysts. Therefore, the greater the catalyst surface area typically results in more catalytic material exposed to the reactants, which further typically results in less time and catalytic material needed to maintain a high rate of productivity.
Alumina (Al2O3) is a well known support for many catalyst systems. It is also well known that alumina has a number of crystalline phases, for example alpha alumina (often noted as α-alumina or α-Al2O3), gamma alumina (often noted as γ-alumina or γ-Al2O3) and others. One of the properties of gamma alumina is that it has a very high surface area. This is commonly believed to be because the aluminum and oxygen molecules are in a crystalline structure or form that is typically not densely packed. Unfortunately, when gamma alumina is heated to high temperatures, the structure of the atoms typically collapses, which results in a substantial surface area decrease. The most dense crystalline form of alumina is alpha alumina. Thus, alpha alumina has the lower surface area but is the most stable at high temperatures.
Alumina can be used as supports and/or catalysts for many heterogeneous catalytic processes. Some of these catalytic processes occur under conditions of high temperature, high pressure and/or high water vapor pressure.
It has long been a desire of those skilled in the catalyst support arts to create a form of alumina that has high surface area like gamma alumina and stability at high temperatures like alpha alumina.
Such a catalyst support would have many uses. One such use is in catalytic reactions that produce high temperature water vapor at high partial pressures. Such an environment challenges the hydrothermal stability of alumina supports making the supports more prone to degradation, fragmentation, or other processes that compromise the ability to support catalytic metals. Hydrothermal stability typically comprises the property of resisting morphological and/or structural change in the face of elevated heat and water vapor pressure.
The Fischer-Tropsch process (also called the Fischer-Tropsch reaction or Fischer-Tropsch synthesis) is an example of a process that can generate water vapor of high partial pressure at high temperatures. In the Fischer-Tropsch process, carbon monoxide and hydrogen are converted into a mixture of organic molecules containing carbon and hydrogen. Those organic molecules containing only carbon and hydrogen are known as hydrocarbons. In addition, other organic molecules containing oxygen in addition to carbon and hydrogen, oxygenates, may be formed during the Fischer-Tropsch process. Hydrocarbons having carbons with no ring formation are known as aliphatic hydrocarbons and may include paraffins and/or olefins. High molecular weight paraffins are particularly desirable as the basis of synthetic diesel fuel.
Typically, the Fischer-Tropsch product stream contains hydrocarbons having a range of numbers of carbon atoms, and thus having a range of molecular weights. Thus, the Fischer-Tropsch products produced by conversion of natural gas commonly contain a range of hydrocarbons including gases, liquids and waxes. Depending on the molecular weight product distribution, different Fischer-Tropsch product mixtures are ideally suited to different uses. For example, Fischer-Tropsch product mixtures containing liquids may be processed to yield gasoline, as well as heavier middle distillates. Hydrocarbon waxes may be subjected to an additional processing step for conversion to liquid and/or gaseous hydrocarbons. Thus, in the production of a Fischer-Tropsch product stream for processing to a liquid fuel it is desirable to obtain primarily hydrocarbons that are liquids and waxes, that are nongaseous hydrocarbons (i.e., C5+ hydrocarbons that comprise 5 carbons or more).
The Fischer-Tropsch process is commonly facilitated by a catalyst. Catalysts desirably have the function of increasing the rate of a reaction without being consumed by the reaction. A feed containing carbon monoxide and hydrogen is typically contacted with a catalyst in a reaction zone that may include one or more reactors. In a batch process, the reactor is closed to introduction of new feed and exit of products. In a continuous process, the reactor is open, with an inflow containing feed, termed a feed stream, passed into the reactor and an outflow containing product, termed a product stream, passed out of the reactor.
Common reactors include packed bed (also termed fixed bed) reactors and slurry bed reactors. Originally, the Fischer-Tropsch synthesis was carried out in packed bed reactors. These reactors have several drawbacks, such as temperature control, that can be overcome by gas-agitated agitated slurry reactors or slurry bubble column reactors. Gas-agitated multiphase reactors sometimes called “slurry reactors” or “slurry bubble columns,” operate by suspending catalytic particles in liquid and feeding gas reactants into the bottom of the reactor through a gas distributor, which produces small gas bubbles. As the gas bubbles rise through the reactor, the reactants are absorbed into the liquid and diffuse to the catalyst where, depending on the catalyst system, they are typically converted to gaseous and liquid products. The gaseous products formed enter the gas bubbles and are collected at the top of the reactor. Liquid products are recovered from the suspending liquid by using different techniques like filtration, settling, hydrocyclones, magnetic techniques, etc. Gas-agitated multiphase reactors or slurry bubble column reactors (SBCRs) inherently have very high heat transfer rates; therefore, reduced reactor cost and the ability to remove and add catalyst online are principal advantages of such reactors in Fischer-Tropsch synthesis, which is exothermic. Sie and Krishna (Applied Catalysis A: General 1999, 186, p. 55), incorporated herein by reference in its entirety, give a history of the development of various Fischer-Tropsch reactors.
Typically, in the Fischer-Tropsch synthesis, the distribution of weights that is observed such as for C5+ hydrocarbons, can be described by likening the Fischer-Tropsch reaction to a polymerization reaction with an Anderson-Shultz-Flory chain growth probability (α) that is independent of the number of carbon atoms in the lengthening molecule. α is typically interpreted as the ratio of the concentration of Cn+1 product to the concentration of Cn product. A value of α at least 0.72 is preferred for producing high carbon-length hydrocarbons, such as those of diesel fractions.
The composition of a catalyst influences the relative amounts of hydrocarbons obtained from a Fischer-Tropsch catalytic process. Common catalysts for use in the Fischer-Tropsch process contain at least one metal from Groups 8, 9, or 10 of the Periodic Table (in the new IUPAC notation, as found in, for example, the CRC Handbook of Chemistry and Physics, 81rst Edition, D. R. Lide, Ed., CRC Press, Inc., 2000–2001, and used throughout this specification as the reference for all element group numbers).
Cobalt metal is particularly desirable in catalysts used in converting natural gas to heavy hydrocarbons suitable for the production of diesel fuel. Alternatively, iron, nickel, and ruthenium have been used in Fischer-Tropsch catalysts. Nickel catalysts favor termination and are useful for aiding the selective production of methane from syngas. Iron has the advantage of being readily available and relatively inexpensive but the disadvantage of a water-gas shift activity by which carbon monoxide reacts with by-product water to produce hydrogen and carbon dioxide. Ruthenium has the advantage of high activity but unfortunately is quite expensive. Consequently, although ruthenium is not the economically preferred catalyst for commercial Fischer-Tropsch production, it is often used in low concentrations as a promoter with cobalt as the catalytic metal.
Thus, catalysts often further employ a promoter in conjunction with the principal catalytic metal. A promoter typically improves a measure of the performance of a catalyst, such as productivity, lifetime, selectivity, reducibility, or regenerability. Further, in addition to the catalytic metal, a Fischer-Tropsch catalyst includes a alumina support material, e.g., alumina and/or other refractory oxides.
The method of preparation of a catalyst may influence the performance of the catalyst in the Fischer-Tropsch reaction. In a common method of loading a Fischer-Tropsch metal to an alumina support, the alumina support is impregnated with a solution containing a dissolved metal-containing compound. When a promoter is used, an impregnation solution may further contain a promoter-containing compound. After drying the support, the resulting catalyst precursor is calcined, typically by heating in an oxidizing atmosphere, to decompose the metal-containing compound to a metal oxide. The catalyst precursor is then typically reduced in hydrogen to convert the oxide compound to reduced “metallic” metal. When the catalyst includes a promoter, the reduction conditions may cause reduction of the promoter, or the promoter may remain as an oxide compound. Despite the vast knowledge of preparation techniques, there is ongoing effort for improving methods of catalyst preparation. For instance, drawbacks to using the alumina supports and/or other refractory oxides include attrition of the supports during Fischer-Tropsch reaction conditions and attenuation of the Fischer-Tropsch catalyst conversion activity.
Consequently, there is a need for a hydrothermally-stable high surface area support for use in the catalytic production of hydrocarbons. Further needs include an improved alumina support. Additional needs include a Fischer-Tropsch catalyst having improved conversion activity and stability. In addition, existing needs include an improved process for the production of hydrocarbons using an alumina supported catalyst.